Each component in a receiver front-end contributes noise to the overall system. The noise of a component can be characterized by its noise factor (F), which is given by the ratio of the signal-to-noise ratio (SNR) at the input of the component to the SNR at the output of the component:F=SNRIN/SNROUT The noise of the receiver front-end increases from input to output as noise from successive components compound. In general, the overall noise factor of the receiver front-end is proportional to the sum of each component's noise factor divided by the cascaded gain of preceding components and is given by:
      F    TOTAL    =            F      1        +                            F                      2            -            1                          -        1                    A        1              +                            F                      3            -            1                          -        1                              A          1                ⁢                  A          2                      +    …    +                            F                      n            -            1                          -        1                              A          1                ⁢                  A          2                ⁢                                  ⁢        …        ⁢                                  ⁢                  A                      n            -            1                              where Fn and An represent the noise factor and gain of the nth component in the receiver front-end, respectively. The above equation reveals that the noise factor and gain of the first gain component (i.e., F1 and A1, respectively) can have a dominant effect on the overall noise factor of the receiver front-end since the noise contributed by each successive component is diminished by the cascaded gain of the components that precede it.
To provide adequate sensitivity, therefore, it is important to keep the noise factor low and the gain high of the first gain component in the receiver front-end. The sensitivity of the receiver front-end determines the minimum signal level that can be detected and is limited by the overall noise factor of the receiver front-end. Thus, in typical receiver designs the first gain component in the front-end is a low-noise amplifier (LNA), which can provide high gain, while contributing low noise to the overall receiver front-end.
Wireless receiver front-ends commonly demand low power consumption to provide, for example, a longer battery life in battery powered devices. But low power makes components in the wireless receiver front-end noisier. In order to overcome the increased noise associated with low power components that follow an LNA, the LNA can raise its voltage gain. The voltage gain of the LNA can be increased without using more current (and thus without burning more power) by increasing the output impedance of the LNA.
However, such an increase in LNA voltage gain has its own associated drawbacks. In particular, LNAs provide relatively linear gain for small input signals, but for large input signals, LNAs can exhibit non-linear behavior in the form of gain compression and noise folding in band. In the presence of a large blocker the desired signal can experience less amplification due to gain compression and also low frequency noise from supply and bias blocks can mix with the high frequency blocker due to LNA non-linearities and fold in band, resulting in a degraded signal-to-noise ratio. Thus, by increasing an LNA's output impedance to increase the LNA's voltage gain, the LNA becomes more susceptible to non-linear behavior due to blockers.
The present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.